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From "Bill Mitchell (JIRA)" <>
Subject [jira] Commented: (AXIS2C-884) Seg fault in libxml when svc client torn down in a multithreaded client
Date Mon, 14 Jan 2008 20:16:34 GMT


Bill Mitchell commented on AXIS2C-884:

One reason it is difficult to isolate exactly what is happening is that WindowsXP exhibits
different behavior in a multiple CPU environment than it does in a single CPU environment.
 In a single CPU environment, using axis2_options_set_xml_parser_reset to disable the xmlCleanupParser()
call on each service termination may succeed in avoiding the libxml seg faults.  This does
not work in the multiple CPU environment, as seen in the crash below.  This crash can happen
even when only one service is created and there is no parser reset call, provided that the
service is created in a "transient" thread that no longer exists when the service is freed.

 	[Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]	
>	msvcr80d.dll!_nh_malloc_dbg(unsigned int nSize=12, int nhFlag=0, int nBlockUse=0, const
char * szFileName=0x0000239c, int nLine=40696612)  Line 268 + 0x15 bytes	C++
 	msvcr80d.dll!malloc(unsigned int nSize=16809748)  Line 154 + 0x15 bytes	C++
 	libxml2.dll!xmlGetGlobalState()  Line 570	C
 	libxml2.dll!__xmlDefaultSAXHandler()  Line 821 + 0x5 bytes	C
 	libxml2.dll!xmlFreeParserCtxt(_xmlParserCtxt * ctxt=0x02ac6b88)  Line 1691 + 0xd bytes	C
 	libxml2.dll!xmlFreeTextReader(_xmlTextReader * reader=0x02a81898)  Line 2196 + 0xc bytes
 	axis2_parser.dll!axis2_libxml2_reader_wrapper_free(axiom_xml_reader * parser=0x02ade048,
const axutil_env * env=0x019cb498)  Line 512 + 0x9 bytes	C
 	axis2_parser.dll!axiom_xml_reader_free(axiom_xml_reader * parser=0x02ade048, const axutil_env
* env=0x019cb498)  Line 35	C
 	axiom.dll!axiom_stax_builder_free(axiom_stax_builder * om_builder=0x02b07938, const axutil_env
* env=0x019cb498)  Line 885	C
 	axiom.dll!axiom_soap_builder_free(axiom_soap_builder * soap_builder=0x02a875e8, const axutil_env
* env=0x019cb498)  Line 189	C
 	axiom.dll!axiom_soap_envelope_free(axiom_soap_envelope * soap_envelope=0x02a85a58, const
axutil_env * env=0x019cb498)  Line 168	C
 	axis2_engine.dll!axis2_msg_ctx_free(axis2_msg_ctx * msg_ctx=0x02aa5e18, const axutil_env
* env=0x019cb498)  Line 374	C
 	axis2_engine.dll!axis2_op_ctx_free(axis2_op_ctx * op_ctx=0x02aea0a0, const axutil_env *
env=0x019cb498)  Line 165	C
 	axis2_engine.dll!axis2_op_client_free(axis2_op_client * op_client=0x02aa3ca8, const axutil_env
* env=0x019cb498)  Line 615	C
 	axis2_engine.dll!axis2_svc_client_free(axis2_svc_client * svc_client=0x019f5028, const axutil_env
* env=0x019cb498)  Line 1287	C
 	axis2_engine.dll!axis2_stub_free(axis2_stub * stub=0x019cb760, const axutil_env * env=0x019cb498)
 Line 129	C

I have not delved into the lower level Windows code to see if the thread management is doing
something special in the multiple CPU case, or if this is just a race condition that appears
more easily in a multi-CPU environment.  

Evidently, the only way to avoid the crash is to create Axis initialization and termination
entry points, as suggested above, that can be called by the application in a persistent thread,
or to build enough functionality into guththila that it can replace libxml.  

As it stands today, for the multithreaded application to avoid the crash it must perform the
xmlInitParser() call to libxml2 itself when it starts up, before multithreaded processing
begins, and also set_xml_parser_reset false in the options of every service client.  

> Seg fault in libxml when svc client torn down in a multithreaded client
> -----------------------------------------------------------------------
>                 Key: AXIS2C-884
>                 URL:
>             Project: Axis2-C
>          Issue Type: Bug
>          Components: core/deployment
>    Affects Versions: Current (Nightly)
>         Environment: Windows XP, Visual Studio 2005, libxml 2.6.25 and libxml 2.6.30,
>            Reporter: Bill Mitchell
>         Attachments: axis2.trace, desc_builder_diff.txt
> In a multithreaded application, if the stub/svc client is freed in a thread different
from that used when the svc client was built, libxml crashes.  The trace below shows the information
available from a release build with debug information embedded.  
> I have verified this is not an effect of combining debug and release C runtimes, or different
versions of the C runtime.  Rebuilding all the libxml related dlls with the same runtime as
is used for Axis and the client app does not solve the problem.  
> Interestingly, rebuilding libxml with threads disabled does make the crash go away. 
But the default build of libxml commonly available has native threads enabled, and building
without thread support may make the library not thread safe.  
> By adding debug trace statements in the axis2.trace file, I have verified that the xml_reader
being torn down when the crash happens is the one used to read the axis2.xml file when the
configuration was first read.  (axis2.trace file attached.)
> Looking at the code in libxml, it appears that libxml decides to close the reader using
an internal close routine intended for closing compressed channels through zlib.  Apparently
the C runtime library returns a -1 EOF status when closing a file opened for read.  The close
routine, gzio.c in zlib, treats this as an error, and when libxml attempts to report the error
and determines that it is in a different thread, things really go downhill fast.  I have not
isolated why the EnterCriticalSection call crashes in the system, but it does.  
> One way to avoid the problem would be to guarantee that the stub/svc client is freed
in the same thread as created it.  In my multithreaded client application, though, I work
hard to share the stub across threads deliberately to reduce the number of distinct service
clients and the associated demand on the server.  
> Windows call traceback at time of crash:
>  	ntdll.dll!7c918fea() 	
>  	[Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]	
>  	msvcr80.dll!78134d09() 	
>  	ntdll.dll!7c910e91() 	
>  	ntdll.dll!7c9106eb() 	
>  	msvcr80.dll!78134d83() 	
>  	ntdll.dll!7c90104b() 	
> >	libxml2.dll!xmlGetGlobalState()  Line 570	C
>  	libxml2.dll!__xmlLastError()  Line 709 + 0x5 bytes	C
>  	libxml2.dll!__xmlRaiseError(void (void *, _xmlError *)* schannel=0x00000000, void (void
*, const char *, <no type>)* channel=0x00000000, void * data=0x00000000, void * ctx=0x00000000,
void * nod=0x00000000, int domain=8, int code=0, xmlErrorLevel level=XML_ERR_ERROR, const
char * file=0x00000000, int line=0, const char * str1=0x00c5d420, const char * str2=0x00000000,
const char * str3=0x00000000, int int1=0, int col=0, const char * msg=0x00c5d360, ...)  Line
452 + 0x5 bytes	C
>  	libxml2.dll!__xmlSimpleError(int domain=8, int code=0, _xmlNode * node=0x00000000,
const char * msg=0x00c5d360, const char * extra=0x00c5d420)  Line 657 + 0x2d bytes	C
>  	libxml2.dll!__xmlIOErr(int domain=8, int code=0, const char * extra=0x00c5d420)  Line
417 + 0x1a bytes	C
>  	libxml2.dll!xmlGzfileClose(void * context=0x03301f80)  Line 1155 + 0x10 bytes	C
>  	libxml2.dll!xmlFreeParserInputBuffer(_xmlParserInputBuffer * in=0x03304578)  Line 2207
+ 0x5 bytes	C
>  	libxml2.dll!xmlTextReaderClose(_xmlTextReader * reader=0x03304760)  Line 2244 + 0x6
bytes	C
>  	axis2_parser.dll!axis2_libxml2_reader_wrapper_free(axiom_xml_reader * parser=0x03301e00,
const axutil_env * env=0x031aa150)  Line 510	C
>  	axiom.dll!axiom_stax_builder_free(axiom_stax_builder * om_builder=0x03271aa8, const
axutil_env * env=0x031aa150)  Line 886	C
>  	axis2_engine.dll!axis2_desc_builder_free(axis2_desc_builder * desc_builder=0x03301d58,
const axutil_env * env=0x031aa150)  Line 141	C
>  	axis2_engine.dll!axis2_conf_builder_free(axis2_conf_builder * conf_builder=0x03301d70,
const axutil_env * env=0x031aa150)  Line 128	C
>  	axis2_engine.dll!axis2_dep_engine_free(axis2_dep_engine * dep_engine=0x031aa2e8, const
axutil_env * env=0x031aa150)  Line 380	C
>  	axis2_engine.dll!axis2_conf_free(axis2_conf * conf=0x0330cad8, const axutil_env * env=0x031aa150)
 Line 328	C
>  	axis2_engine.dll!axis2_conf_ctx_free(axis2_conf_ctx * conf_ctx=0x00000000, const axutil_env
* env=0x031aa150)  Line 439	C
>  	axis2_engine.dll!axis2_svc_client_free(axis2_svc_client * svc_client=0x031aa1a8, const
axutil_env * env=0x031aa150)  Line 1303	C
>  	axis2_engine.dll!axis2_stub_free(axis2_stub * stub=0x031aa198, const axutil_env * env=0x031aa150)
 Line 131	C

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